Our long-term objective is to understand the role of titin in cardiac function. Titin is a giant elastic protein that spans the length of the half sarcomere and forms, in addition to the thick and thin filaments, the third myofilament system of striated muscle. Upon stretch of the sarcomere, titin extends and develops a force that is a primary contributor to the overall passive force of muscle. Titin-based passive force maintains sarcomeric integrity during contraction and underlies a large portion of the diastolic force of the heart. In this work we propose to investigate novel functions of titin with the central hypothesis: titin modulates actomyosin interaction. Although rapid progress is being made in understanding titin's role in passive force generation, this role is typically assumed to be independent of actomyosin interaction (which underlies active force) and independent of the physiological state of muscle. However, several new findings begin to challenge the traditional view of titin as simply a passive mechanical component of the heart. For example, preliminary studies from our laboratory suggest that titin may modulate the calcium-sensitivity of active force and thereby underlie part of the Frank-Starling mechanism of the heart. The proposed work aims to elucidate this putative interplay between titin and active force via mechanical experiments on cardiac myocytes, and X-ray diffraction experiments on cardiac muscle. A further goal is to investigate how titin's role in modulating active force my be regulated. Our hypothesis is that regulation may be achieved via expressing length variants of titin's extensible subsegments and/or via the binding of ligands to, and phosphorylation of, these subsegments. For this purpose we will study the length-sensitivity of force development of myocytes that express varying ratios of titin isoforms. We will also use atomic force microscopy to study the extensibility of titin's subsements that are differentially expressed in different isoforms, as well as the effects of ligands and phosphorylation on their extensibility. We anticipate tht the proposed work will increase our understanding of titin and particularly of its interplay with actomyosin interaction. This is expected to result in a more complete understanding of muscle contraction and to lay the basis for a better understanding of muscle disease.